Many hydraulic systems include multiple actuators. The actuators are powered by hydraulic fluid supplied from a hydraulic fluid source, such as a pump. As used throughout this description, the words “power” in its various forms when referring to the actuators means to act on the actuators so as to cause movement or actuation, or attempt to cause movement or actuation. One or more valves associated with each actuator control the flow of fluid to and from the actuator. Often, such as in mobile equipment, the multiple actuators are powered simultaneously for performing various functions. For example, in an excavator, an operator may simultaneously power actuators associated with the swing, the arm, and the boom. The loads acting on each actuator differ dependent upon many variables. The pressure for powering the actuators differs dependent upon the load. To power multiple actuators simultaneously, when the actuators are subjected to different loads, it is desirable for the pump to provide sufficient flow and pressure to allow control of all of the actuators. Generally speaking, the valve (or valves) associated with each actuator is controlled to vary the resistance to flow. In the simplest circuits, this allows the valve to control the direction and speed of its associated actuator. In more complex circuit with multiple valve and actuator pairings, the valves commonly are controlled to prevent any one pairing to offer too little resistance, which would result in a reduction in supply pressure below that needed to power the other actuators.
At times, the pump is incapable of maintaining the system pressure at a level for powering all of the actuators at the speeds commanded by the operator. When this occurs, it is desirable to maintain the commanded speed relationships among the various actuators. For example, if the operator of an excavator desires the arm to move at a rate twice that of the boom, it is desirable for this relationship to be maintained even when the pump is incapable of maintaining the pressure for powering the arm and the boom actuators at the speeds commanded by the operator.